1. Field of the Invention
The present invention relates to a thermal printer.
2. Description of the Related Art
A thermal printer provided with a heat-sensitive printing section including a thermal head and a platen has a relatively small number of parts and is easily downsized, so that the thermal printer has been widely employed as a printer attached to a cash register, a portable terminal unit, an ATM, etc. Regarding this kind of thermal printer, a printer having a platen that functions as a back-up roller to realize stable printing on a printing sheet (or heat-sensitive paper) and also functions as a drive roller to continuously supply the printing sheet by frictional force, has been known as a compact printer that does not require an exclusive mechanism for supplying a printing sheet.
FIG. 22 exemplary shows several components of a conventional thermal printer having the above-described drive-roller platen. This thermal printer 200 includes a thermal head 202, a platen 204 cooperating with the thermal head 202 to nip a printing sheet P therebetween, a plate spring 206 elastically pressing the thermal head 202 against the platen 204, a drive mechanism 208 rotationally driving the platen 204, a frame unit 210 supporting the thermal head 202 and the platen 204, a control circuit board 212 electrically connected to the thermal head 202 and drive mechanism 208, and a casing (not shown) accommodating these components as well as a battery 214 in a suitable relative layout. Further, a head release lever (not shown) may be provided to shift the thermal head 202 against the biasing force of the plate spring 206, in order to facilitate the insertion of the printing sheet P between the thermal head 202 and the platen 204.
The thermal head 202 is structured by arranging a heat generating element on the surface of a substrate made of a hard material such as ceramic, and fixing the substrate to a metallic supporting plate having a reinforcing and heat-radiating function. The thermal head 202 is pivotably supported on the frame unit 210 via a shaft provided on the supporting plate. The platen 204 is rotatably supported by the frame unit 210 via a shaft and is driven by the drive mechanism 208 for rotation, to continuously feed the printing sheet P unrolled from a sheet roll R with the printing sheet P sliding between the thermal head 202 and the platen 204 under pressure. During this period, the heat generating element provided on the surface of the substrate is operated electrically, so that the thermal head 202 executes a desired printing onto the printing sheet P. The plate spring 206 generates a required level of contact pressure between the thermal head 202 and the platen 204 to absorb any dimensional and positional error of the head 202 and platen 204 and to realize a stable printing while following a change in the thickness of the printing sheet P. The drive mechanism 208 includes a rotation drive source 216, and a power transmission mechanism (not shown) for transmitting an output torque of the rotation drive source 216 to the platen 204. The control circuit board 212 is generally connected to the thermal head 202 and the rotation drive source 216 via a flexible wiring board 218.
The above-described thermal printer having the drive platen can be made more compact by omitting a mechanism exclusively acting to feed the printing sheet, so that it is expected that the thermal printer can be used in connection with various kinds of portable information apparatuses or hand-held operable devices, such as an electronic notebook, a personal digital assistance (PDA), a mobile phone, and the like. However, in the conventional thermal printer with the driving platen, it has been difficult to reduce the external dimensions of the thermal printer to a level suitable for use in the portable information apparatuses, because of a relative layout of the components incorporated in the printer.
Specifically, in the conventional thermal printer 200 having a general structure as shown in FIG. 22, the thermal head 202 is placed inside the frame unit 210 via the plate spring 206, to lean laterally against the platen 204 supported by the frame unit 210. Further, the rotation drive source 216 is located behind the frame unit 21, and the control circuit board 212 connected with the flexible wiring board 218 is disposed further behind the drive source. The conventional thermal printer incorporating these components in this relative layout has a difficulty in reducing the dimensions, particularly in the height and length (or depth) directions, of the casing to a level permitting it to be carried together with anyone of the above-described various information apparatuses.
In this respect, it may be appreciated that each of the essential components of the thermal printer has a dimension necessary and enough to exhibit its own required function, so that a thoughtless reduction in dimensions of the components, for the purpose of facilitating the reduction in dimensions of the printer, may have a risk of causing other inconveniences such as a degradation of performance. Further, it is a concern that the reduction in dimensions of the thermal printer may make it difficult to quickly set a printing sheet in a printable state.